1. Groups of ISA Brown pullets were transferred from 8- to 16-h photoperiod
s at 34, 44 or 54 d. In each group, 12 birds were injected on alternate day
s over a 12-d period starting 6 d before the change in photoperiod with bet
a -oestradiol-3-benzoate (1 mg/kg body weight) or with arachis oil vehicle
(controls). Short-day controls were similarly injected from 28 to 40 d. Lon
g-day (16 h) controls were also included in the trial but were not injected
. Age at first egg (AFE) was recorded and plasma luteinising hormone ( LH)
concentrations were measured around the time of oestradiol treatment.
2. Mean AFE for birds photostimulated at 34 d was not significantly differe
nt from short-day controls. Birds photostimulated at 44 and 54 d matured at
similar ages but 3 weeks earlier than short-day controls (P< 0.05).
3. There was a tendency for oestradiol to advance AFE for birds photostimul
ated at 34 d (P=0.15) but to delay AFE following photostimulation at 44 d (
P=0.23). Oestradiol significantly delayed AFE for the birds photostimulated
at 54 d (P=0.01).
4. Plasma LH levels during 6 d of oestradiol injection but before transfer
from 8- to 16-h photoperiods tended to fall between 28 and 34 d, were relat
ively constant between 38 and 44 d, but declined significantly between 48 a
nd 54 d. Following photostimulation at 34 d, increases in plasma LH levels
for oestradiol-injected birds were significantly greater than for controls.
Oestradiol treatment had no significant effect on changes in plasma LH con
centrations after photostimulation at 44 or 54 d.
5. This trial confirms previous work showing that pullets are unresponsive
to photostimulation before 6 weeks of age. It also demonstrates that raisin
g circulating oestrogen levels by injecting 0.5 mg/kg oestradiol benzoate o
n alternate days enhances the LH response to photostimulation at 34-d, but
only very slightly sensitises a 34-d old bird to an increase in photoperiod
which, 10 d later, is capable of advancing AFE in control birds by 24 d. I
ncreased circulating oestrogen might be a factor which allows pullets to ad
vance AFE in response to an increase in daylength.